This proposal has arisen from a 9-year old study of transmitter choice in cultured sympathetic principal neurons of the rat. When developing in certin culture conditions, these neurons can be shifted from adrenergic to cholinergic status (a new form of neuronal plasticity). To study this transition in developing neurons, we devised a microculture method which permits the electrophysiologically-characterized transmitter status of a single neuron to be correlated directly with the fine structure of its endings or its immunoreactivity. With this method we found that during the transition, the neurons display adrenergic:cholinergic dual function, and that the transition can be followed directly by recording repeatedly from the same neuron over periods up to 45d. This method also revealed that some neurons secreted another transmitter, probably adenosine, with one of the amines. Neurons dissociated from adult ganglia also sometimes display adrenergic:cholinergic dual function. Parallel studies of primary sensory neurons and myenteric neurons are now well underway. Recently, several laboratories have reported tha sympathetic, sensory, and myenteric neurons of adult rats disply immunoreactivity for many active peptides (5 in sympathetic neurons alone); there is evidence for secretion of several, nd for amine:peptide dual function. This indicates that transmitter choice and its control are considerably more complex in these types of neurons than previously supposed. Since the culture methods mentioned above permits a direct study of synaptic functions, including multiple-transmitter status, which is often difficult in vivo, we have begun investigation of peptidergic functions in our cultures. We have preliminary evidence for Substance P-like and Avian Pancreatic Polypeptide-like immunoreactivity. In this proposal we request support for a systematic investigation of peptidergic functions in our cultured peripheral neurons. Our specific aims are: i) With conventional immunocytochemical methods to screen four types of cultured neurons (sympathetic, sensory, myenteric and parasympathetic) for peptides reported in vivo. ii) To co-culture the neurons with target cells sensitive to the peptides, in the novel microcultures, and to correlate peptidergic functon, fine structure and immunoreactivity in single neurons. iii) To investigate the control of expression of peptidergic functions as we have previously done with adrenergic:cholinergic functions. The long-term objective is to enlarge understanding of transmitter choice, a fundamental aspect of neuronal cell biology.